Fuel in an aircraft is typically stored in one or more fuel tanks or sealed compartments located in the interior space of the wings and fuselage of the aircraft. A fuel delivery system includes one or more fuel pumps arranged to draw fuel from the fuel tank(s) and deliver fuel to the engine(s).
The location of the fuel pump within a fuel compartment can be restricted, for example by the irregular shape of the fuel compartment, internal structural components, or the physical size of the fuel pump. When the fuel pump is not located at the lowermost point in the compartment, the fuel pump is unable to empty the fuel compartment of fuel. Conventional aircraft fuel systems therefore suffer from the problem of unusable fuel (also known as residual fuel)—that is, fuel which cannot be fed out of the fuel tank because it is trapped in a low or inaccessible region.
The weight of unusable/residual fuel is included in the empty weight of the aircraft. This extra empty weight reduces the aircraft operator's permitted usage of the aircraft in terms of range, passengers and cargo.